Transparent polyester film having at least three layers and process for its production

ABSTRACT

A transparent, biaxially oriented polyester film is disclosed, with a base layer B which comprises at least 80% by weight of thermoplastic polyester, and with at least one intermediate layer Z, and with at least one outer layer A. The outer layer A is composed of a polymer or of a mixture of polymers which contains at least 85% by weight of ethylene 2,6-naphthalate units and up to 15% by weight of ethylene terephthalate units, and/or up to 15% by weight of units derived from cycloaliphatic or aromatic diols and/or dicarboxylic acids, and the intermediate layer Z is composed of a mixture of polymers which contains at least 3% by weight of ethylene 2,6-naphthalate units and up to 97% by weight of ethylene terephthalate units, and/or up to 97% by weight of units derived from cycloaliphatic or aromatic diols and/or dicarboxylic acids. The T g   2  value of the polyester film is above the T g   2  value of the base layer B, but below the T g   2  value of the outer layer A. The film has low permeability to atmospheric oxygen and exhibits very good adhesion between the individual layers. It is particularly suitable for packaging purposes, specifically for packaging foods or other consumable items.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a transparent, biaxially orientedpolyester film with a base layer B which comprises at least 80% byweight of thermoplastic polyester, and with at least one intermediatelayer Z, and with at least one outer layer A. The invention furtherrelates to the use of the film and to a process for its production.

Prior Art

[0002] EP-A-0 878 297 describes a transparent, biaxially orientedpolyester film with a base layer B, at least 80% by weight of which iscomposed of a thermoplastic polyester, and with at least one outer layerA which is composed of a mixture of polymers which contains at least 40%by weight of ethylene 2,6-naphthalate units (PEN) and up to 40% byweight of ethylene terephthalate units (PET) and/or up to 60% by weightof units derived from cycloaliphatic or aromatic diols and/ordicarboxylic acids.

[0003] If the outer layer A of the film of EP-A-0 878 297 contains highconcentrations of ethylene 2,6-naphthalate units, the film has atendency for delamination between the outer layer A and the base layerB. If, on the other hand, the outer layer A contains low concentrationsof ethylene 2,6-naphthalate units, the thickness of this layer has to beraised in order to achieve the desired low oxygen permeation of not morethan 80 cm³/(m²·bar·d).

[0004] In a film in Example 8 of EP-A-0 878 297 the outer layer A usespure polyethylene 2,6-naphthalate (corresponding to 100% by weight ofethylene 2,6-naphthalate units). In this case there is no significantadhesion between the outer layer A and the base layer B. The film isunsuitable for industrial use (e.g. as a composite film), since the bondreleases even when subjected to a low level of mechanical stress, due tothe low adhesion between the outer layer A and the base layer B of thepolyester film.

[0005] In a film in Example 11 of EP-A-0 878 297, the outer layer Acontains 60% by weight of ethylene 2,6-naphthalate units. In order toachieve the low oxygen permeation demanded, below 80 cm³/(m²·bar·d), thethickness of the outer layer A has to be raised to 3 μm, and this iseconomically disadvantageous (high capital expenditure and high materialcosts).

[0006] U.S. Pat. No. 5,795,528 describes a coextruded film laminatewhich has alternating layers of PEN and PET. Like the film of EP-A-0 878297, this film has a tendency toward delamination between the individuallayers of PEN and PET. There is no significant adhesion between theselayers. A laminate of this type is therefore again unsuitable forindustrial use.

[0007] It was an object of the present invention, therefore, to providea transparent, biaxially oriented polyester film which overcomes thedisadvantage of the prior art films and in particular has improvedadhesion between the individual layers. It should be simple andcost-effective to produce, have good barrier properties, and pose noproblems of disposal.

SUMMARY OF THE INVENTION

[0008] The object is achieved by means of a transparent, biaxiallyoriented polyester film with a base layer B which comprises at least 80%by weight of thermoplastic polyester, with at least one intermediatelayer Z, and with at least one outer layer A, the characterizingfeatures of which are regarded as being that

[0009] the outer layer A is composed of a polymer, of a mixture ofpolymers/copolymers, or of a copolymer, which contains at least 85% byweight of ethylene 2,6-naphthalate units and up to 15% by weight ofethylene terephthalate units, and/or up to 15% by weight of unitsderived from cycloaliphatic or aromatic diols and/or dicarboxylic acids;

[0010] the intermediate layer Z is composed of a mixture ofpolymers/copolymers, or of a copolymer, which contains at least 3% byweight of ethylene 2,6-naphthalate units, and up to 97% by weight ofethylene terephthalate units, and/or up to 97% by weight of unitsderived from cycloaliphatic or aromatic diols and/or dicarboxylic acids,and the T_(g)2 value of the polyester film is above the T_(g)2 value ofthe base layer B but below the T_(g)2 value of the outer layer A.

[0011] The film of the invention has low oxygen permeation, below 85cm³/(m²·bar ·d), and minimum adhesion (between the individual layers)greater than 0.5 N/25 mm.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The structure of the film of the invention has at least threelayers, and is then composed of the outer layer A, of a base layer B,and of an intermediate layer Z located between the outer layer A and thebase layer B.

[0013] Preference is given to a polyester film in which the polymers ofthe outer layer A contain at least 90% by weight of ethylene2,6-naphthalate units and up to 10% by weight of ethylene terephthalateunits. Among these, particular preference is in turn given to apolyester film in which the polymers of the outer layer A contain atleast 92% by weight of ethylene 2,6-naphthalate units and up to 8% byweight of ethylene terephthalate units. However, the outer layer A mayalso be composed entirely of ethylene 2,6-naphthalate polymers.

[0014] Preference is also given to a polyester film in which thepolymers of the intermediate layer Z contain at least 5% by weight ofethylene 2,6-naphthalate units and up to 95% by weight of ethyleneterephthalate units. Among these, particular preference is in turn givento a polyester film in which the polymers of the intermediate layer Zcontain at least 7% by weight of ethylene 2,6-naphthalate units and upto 93% by weight of ethylene terephthalate units.

[0015] Examples of suitable aliphatic diols are diethylene glycol,triethylene glycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH,where n is an integer from 3 to 6 (in particular 1,3-propanediol,1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol), or branchedaliphatic glycols having up to 6 carbon atoms, and cycloaliphatic diolshaving one or more rings and if desired containing heteroatoms. Amongthe cycloaliphatic diols, mention may be made of cyclohexanediols (inparticular 1,4-cyclohexanediol). Examples of other suitable aromaticdiols are those of the formula HO—C₆H₄—X—C₆H₄—OH where X is —CH₂—,—C(CH₃)₂—, —C(CF₃)₂—, —O—, —S— or —SO₂—. Besides these, bisphenols ofthe formula HO—C₆H₄—C₆H₄—OH are also very suitable.

[0016] Preferred aromatic dicarboxylic acids are benzenedicarboxylicacids, naphthalenedicarboxylic acids (for example naphthalene-1,4- or-1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylic acids (in particularbiphenyl-4,4′-dicarboxylic acid), diphenylacetylene-x,x′-dicarboxylicacids (in particular diphenylacetylene-4,4′-dicarboxylic acid) orstilbene-x,x′-dicarboxylic acids. Among the cycloaliphatic dicarboxylicacids, mention may be made of cyclohexanedicarboxylic acids (inparticular cyclohexane-1,4-dicarboxylic acid). Among the aliphaticdicarboxylic acids, the C₃-C₁₉-alkanedioic acids are particularlysuitable, where the alkane moiety may be straight-chain or branched.

[0017] The base layer of the film is preferably composed of at least 90%by weight of the thermoplastic polyester. Polyesters suitable for thisare those made from ethylene glycol and terephthalic acid (=polyethyleneterephthalate, PET), from ethylene glycol andnaphthalene-2,6-dicarboxylic acid (=polyethylene 2,6-naphthalate, PEN),from 1,4-bishydroxymethylcyclohexane and terephthalic acid(=poly-1,4-cyclohexanedimethylene terephthalate, PCDT), and also fromethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl-4,4′-dicarboxylic acid (polyethylene 2,6-naphthalatebibenzoate, PENBB). Particular preference is given to polyesters whichare composed of at least 90 mol %, preferably at least 95 mol %, ofethylene glycol units and terephthalic acid units or of ethylene glycolunits and naphthalene-2,6-dicarboxylic acid units. The remaining monomerunits are derived from other diols and/or dicarboxylic acids. Examplesof suitable diol comonomers are diethylene glycol, triethylene glycol,aliphatic glycols of the formula HO—(CH₂)_(n)—OH, where n is an integerfrom 3 to 6, branched aliphatic glycols having up to 6 carbon atoms,aromatic diols of the formula HO—C₆H₄—X—C₆H₄—OH where X is —CH₂—,—C(CH₃)₂—, —C(CF₃)₂—, —O—, —S— or —SO₂—, or bisphenols of the formulaHO—C₆H₄—C₆H₄—OH are employed.

[0018] The dicarboxylic acid comonomer units are preferably derived frombenzene-dicarboxylic acids, naphthalenedicarboxylic acids,biphenyl-x,x′-dicarboxylic acids (in particularbiphenyl-4,4′-dicarboxylic acid), cyclohexanedicarboxylic acids (inparticular cyclohexane-1,4-dicarboxylic acid),diphenylacetylene-x,x′-dicarboxylic acids (in particulardiphenylacetylene-4,4′-dicarboxylic acid), stilbene-x,x′-dicarboxylicacid or C₁-C₁₆-alkanedicarboxylic acids, where the alkane moiety may bestraight-chain or branched.

[0019] The polyesters may be prepared by the transesterificationprocess. The starting materials for this are dicarboxylic esters anddiols, which are reacted using the customary transesterificationcatalysts, such as salts of zinc, of calcium, of lithium and ofmanganese. The intermediates are then polycondensed in the presence ofwidely used polycondensation catalysts, such as antimony trioxide ortitanium salts. The preparation may be carried out just as successfullyby the direct esterification process in the presence of polycondensationcatalysts, starting directly from the dicarboxylic acids and the diols.

[0020] The polymers/copolymers for the outer layer A and for theintermediate layer Z may be prepared in three different ways:

[0021] a) In copolycondensation, terephthalic acid andnaphthalene-2,6-dicarboxylic acid are placed in a reactor together withethylene glycol, and polycondensed to give a polyester, using thecustomary catalysts and stabilizers. The terephthalate and naphthalateunits are then randomly distributed in the polyester.

[0022] b) Polyethylene terephthalate (PET) and polyethylene2,6-naphthalate (PEN), in the desired ratio, are melted together andmixed, either in a reactor or preferably in a melt kneader (twin-screwkneader) or in an extruder. Immediately after the melting,transesterification reactions between the polyesters begin. Initially,block copolymers are obtained, but as reaction time increases—dependingon the temperature and mixing action of the agitator—the blocks becomesmaller, and long reaction times give a random copolymer. However, it isnot necessary and also not always advantageous to wait until a randomdistribution has been achieved, since the desired properties are alsoobtained with a block copolymer. The resultant copolymer is thenextruded from a die and granulated.

[0023] c) PET and PEN are mixed as granules in the desired ratio, andthe mixture is fed to the extruder for the outer layer A. Here, thetransesterification to give the copolymer takes place directly duringthe production of the film. This process has the advantage of being verycost-effective, and generally gives block copolymers, the block lengthbeing dependent on the extrusion temperature, the mixing action of theextruder and the residence time in the melt.

[0024] In a preferred embodiment of the invention, from 0.1 to 20% byweight of the polymers of the base layer B are identical with those ofthe outer layer A and with those of the intermediate layer Z. These areeither directly admixed with the base layer B during extrusion or are inany case present in the film due to addition of regrind. The proportionof these copolymers in the base layer is selected in such a way that thebase layer has crystalline character.

[0025] In another advantageous embodiment, the film encompasses, on theside facing away from the outer layer A, another outer layer C ofpolyethylene terephthalate, and this layer comprises pigments.

[0026] The film of the invention has a high oxygen barrier and highadhesion between its individual layers. If, in contrast, the polymersused for the outer layer A contain less than 85% of ethylene2,6-naphthalate units and more than 15% by weight of ethyleneterephthalate units, although the film then has somewhat lesspermeability to oxygen than a standard polyester film (composed of 100%by weight of polyethylene terephthalate), its permeability is still muchtoo high.

[0027] If the copolymers used for the intermediate layer Z contain lessthan 3% by weight of ethylene 2,6-naphthalate units and more than 97% byweight of ethylene terephthalate units, the adhesion between the outerlayer A and the intermediate layer Z becomes inadequate. When subjectedto mechanical stress the film tends toward delamination, which isundesirable and makes the film unusable.

[0028] A difference from the prior art is that in films of the inventionmoreover the glass transition temperature T_(g) of the (co)polymer or ofthe (co)polymers of the outer layer A is higher than the glasstransition temperature T_(g) of the polymers for the intermediate layerZ and for the base layer B. The glass transition temperature T_(g) ofthe (co)polymers used for the outer layer A is preferably in the rangefrom 90 to 120° C. In the DSC (differential scanning calorimetry)determination of the glass transition temperatures T_(g), thetransitions of the layers cannot be differentiated.

[0029] Glass transitions which are determined on biaxially oriented,heat-set films in the first heating procedure (termed T_(g)g1 below)are, due to crystallinity and also to molecular stresses in theamorphous fraction of the specimens, relatively small in size,distributed over a wide temperature range, and shifted to highertemperatures. Because of orientation effects in particular, they are notsuitable for characterizing a polymer. The resolution of DSC analyzersis often insufficient to detect the glass transitions in the firstheating procedure (T_(g)1) of the individual layers of the film of theinvention, the transitions being “blurred” and small, due to orientationand crystallinity.

[0030] If the specimens are melted and then rapidly cooled again tobelow their glass transition temperature (quenched), the orientationeffects are eliminated. On renewed heating, glass transitions(designated T_(g)2 here) are then measured which have a greaterintensity and are characteristic of the respective polymers. However,even here it is not possible to differentiate the glass transitions ofthe individual layers, since the layers mix on melting and thepolyesters present therein enter into transesterification reactions withone another. It is fully sufficient, however, to compare the T_(g)2 ofthe entire coextruded films with the T_(g)2 of the polymer used for thebase layer B. In known films the T_(g)2 value of the base layer ishigher than that of the coextruded film, whereas the T_(g)2 value of theouter layer is lower than that of the base layer and also than that ofthe coextruded film. Exactly the opposite of this applies for the filmof the invention. Here, the T_(g)2 value of the coextruded film ishigher than that of the base layer B but below the T_(g)2 value of theouter layer A.

[0031] The base layer B, the intermediate layer Z, and the outerlayer(s) may also comprise customary additives, such as stabilizers andantiblocking agents. They are expediently added to the polymer or to thepolymer mixture even before melting takes place. Examples of stabilizersare phosphorus compounds, such as phosphoric acid and phosphoric esters.Typical antiblocking agents (also termed pigments in this context) areinorganic and/or organic particles, for example calcium carbonate,amorphous silica, talc, magnesium carbonate, barium carbonate, calciumsulfate, barium sulfate, lithium phosphate, calcium phosphate, magnesiumphosphate, aluminum oxide, LiF, the calcium, barium, zinc and manganesesalts of the dicarboxylic acids used, carbon black, titanium dioxide,kaolin, crosslinked polystyrene particles and crosslinked acrylateparticles.

[0032] The additives selected may also be mixtures of two or moredifferent antiblocking agents or mixtures of antiblocking agents of thesame makeup but of different particle size. The particles may be addedto the individual layers in the customary concentrations, e.g. asglycolic dispersion during polycondensation or via masterbatches duringextrusion. Pigment concentrations of from 0.0001 to 5% by weight haveproven particularly suitable. A detailed description of the antiblockingagents is found, for example, in EP-A-0 602 964.

[0033] The film may be coated and/or corona- or flame-pretreated toestablish other desired properties. Typical coatings are layers whichpromote adhesion, are antistatic, improve slip, or have release action.These additional layers may be applied to the film by in-line coatingusing aqueous dispersions, before transverse orientation. The film isalso particularly suitable for metallizing or coating with ceramicsubstances (SiO_(x), Al₂O₃). Particularly good oxygen barrier values areachieved if the outer layer A is metallized or ceramic-coated.

[0034] The polyester film of the invention preferably also comprises asecond outer layer C. The structure, thickness, and makeup of the secondouter layer C may be selected without reference to the outer layer Aalready present, and the second outer layer C may also comprise theabovementioned polymers or polymer mixtures, but these do not have to beidentical with the chemical makeup of outer layer A. The second outerlayer C may also comprise other commonly used outer layer polymers.

[0035] Between the base layer B and the outer layer A, there is theintermediate layer Z. The thickness of the intermediate layer isgenerally above 0.1 μm and is preferably in the range from 0.2 to 20 μm,particularly preferably in the range from 0.3 to 10 μm.

[0036] The thickness of the outer layer(s) is generally above 0.3 μm,preferably in the range from 0.4 to 5 μm, and particularly preferably inthe range from 0.5 to 4 μm, and the thicknesses of the outer layers maybe identical or different.

[0037] The total thickness of the polyester film of the invention mayvary within wide limits and depends on the application envisaged. It ispreferably from 4 to 100 μm, in particular from 5 to 50 μm, withpreference from 6 to 30 μm, the proportion of the total thickness madeup by the base layer preferably being from about 40 to 90%.

[0038] The present invention also provides a process for producing thisfilm. It encompasses

[0039] a) producing a film having two or more layers from a base layer Band outer layer(s) A and, where appropriate, C, by coextrusion;

[0040] b) biaxially stretching the film, and

[0041] c) heat-setting the stretched film.

[0042] To produce the outer layer A, it is expedient to feed granules ofpolyethylene terephthalate and polyethylene 2,6-naphthalate directly tothe extruder in the desired mixing ratio. At about 300° C., the twomaterials can be melted and can be extruded. Under these conditions,transesterification reactions can occur in the extruder and during thesecopolymers are formed from the respective homopolymers.

[0043] The polymers for the base layer B are expediently fed in viaanother extruder. Any foreign bodies or contamination which may bepresent can be filtered off from the polymer melt before extrusion. Themelts are then extruded through a coextrusion die to give flat meltfilms and are layered one upon the other. The composite film is thendrawn off and solidified with the aid of a chill roll and other rolls ifdesired.

[0044] The biaxial stretching is generally carried out sequentially,stretching first longitudinally (i.e. in the machine direction) and thentransversely (i.e. perpendicularly to the machine direction). This leadsto orientation of the molecular chains within the polyester. Thelongitudinal stretching can be carried out with the aid of two rollersrotating at different rates corresponding to the desired stretchingratio. For the transverse stretching, use is generally made of anappropriate tenter frame.

[0045] The temperature at which the orientation procedure is carried outcan vary over a relatively wide range and depends on the propertiesdesired in the film. In general, the longitudinal stretching is carriedout at from 80 to 130° C., and the transverse stretching at from 90 to150° C. The longitudinal stretching ratio is generally in the range from2.5:1 to 6:1, preferably from 3:1 to 5.5:1. The transverse stretchingratio is generally in the range from 3.0:1 to 5.0:1, preferably from3.5:1 to 4.5:1.

[0046] During the subsequent heat-setting, the film is held for from 0.1to 10 s at a temperature of from 150 to 250° C. The film is then woundup in a conventional manner.

[0047] A great advantage of this process is that it is possible to feedthe extruder with granules, which do not block the machine.

[0048] A further advantage is that the production costs of the film ofthe invention are only insignificantly greater than those of a film madefrom standard polyester raw materials. The other properties of the filmof the invention which are relevant to its processing and its use remainessentially unchanged or have even been improved. It has also beenensured that cut material arising directly in the plant during filmproduction can be used again in the form of regrind for film productionin amounts of up to 60% by weight, preferably from 10 to 50% by weight,based in each case on the total weight of the film, without anysignificant resultant adverse effect on the physical properties of thefilm produced with incorporation of regrind.

[0049] The film of the invention has excellent suitability for packagingfood or other consumable items. It has excellent barrier properties, inparticular with respect to oxygen. It has been assured that theindividual layers of the film remain adhering to one another when thefilm is processed, e.g. to give film laminates, and do not delaminate.

[0050] It is also possible to improve the gloss and the haze of thefilm, compared with prior art films. It has been ensured that regrindcan be reintroduced to the extrusion process during production of thefilm at a concentration of up to 60% by weight, based on the totalweight of the film, without any significant resultant adverse effect onthe physical properties of the film.

[0051] The very good handling properties of the film and its excellentprocessing properties make it particularly suitable for processing onhigh-speed machinery.

[0052] The table below (Table 1) gives the most important filmproperties of the invention again at a glance for quick reference. TABLE1 Range according Particularly to the invention Preferred preferred UnitTest method Outer layer A Ethylene 2,6-naphthalate units >85 >90 >92 %by weight Ethylene terephthalate units <15 <10  <8 % by weightThickness >0.3 0.4-5.0  0.5-4.0  μm Intermediate layer Z Ethylene2,6-naphthalate units >3  >5  >7 % by weight Ethylene terephthalateunits <97 <95 <93 % by weight Thickness >0.1 0.2-20.0 0.3-10.0 μm Filmproperties Oxygen permeation <80 <75 <70 (cm³/m² -bar · d) DIN 53380,Part 3 Adhesion between layers A and Z, and also Z and B >0.5  >0.7 >1.0 N/25 mm internal

Test Methods

[0053] The following methods were utilized to characterize the rawmaterials and the films:

Oxygen Permeability

[0054] The oxygen barrier test took place using a Mocon Modern Controls(USA) OX-TRAN 2/20, as in DIN 53 380, Part 3.

SV (Standard Viscosity)

[0055] The standard viscosity SV (DCA) is measured by analogy with DIN53726, at 25° C. in dichloroacetic acid. The intrinsic viscosity (IV) iscalculated from the standard viscosity as follows

IV[η]=6.907·10⁻⁴ SV (DCA)+0.063096 [dl/g].

Coefficient of Friction

[0056] The coefficient of friction was determined to DIN 53 375. Thecoefficient of sliding friction was measured 14 days after production.

Surface Tension

[0057] Surface tension was determined using what is known as the inkmethod (DIN 53 364).

Haze

[0058] The haze of the film was measured to ASTM-D1003-52. The Hölz hazemeasurement was determined by analogy with ASTM-D1003-52, but, in orderto utilize the ideal measurement range, measurements were taken on fourpieces of film laid one on top of the other, and a 1° slit diaphragm wasused instead of a 4° pinhole.

Gloss

[0059] Gloss was determined to DIN 67 530. The reflectance was measured,this being a characteristic optical value for a film surface. Based onthe standards ASTM-D523-78 and ISO 2813, the angle of incidence was setat 20° or 60°. A beam of light at the set angle of incidence hits theflat test surface and is reflected and/or scattered thereby. Aproportional electrical variable is displayed representing light rayshitting the photoelectric detector. The value measured is dimensionlessand must be stated together with the angle of incidence.

Glass Transition Temperatures

[0060] The glass transition temperatures T_(g)1 and T_(g)2 weredetermined with the aid of DSC (differential scanning calorimetry) onfilm specimens. A DuPont DSC 1090 was used. The heating rate was 20K/min, and the specimen weight was about 12 mg. The glass transitionT_(g)1 was determined in the first heating procedure. Many of thespecimens showed an enthalpy relaxation (a peak) at the beginning of thestep-like glass transition. The temperature taken as T_(g)1 was that atwhich the step-like change in heat capacity—ignoring the enthalpyrelaxation peak—achieved half of its height in the first heatingprocedure. In all cases, there was only a single glass transition stagein the thermogram in the first heating procedure. It is possible thatthe enthalpy relaxation peaks obscured the fine structure of thetransition, or that the resolution of the device was not adequate toseparate the small, “blurred” transitions of oriented, crystallinespecimens. In order to eliminate their heat history the specimens wereheld at 300° C. for 5 minutes after the heating procedure, and thenquenched with liquid nitrogen. The temperature for the glass transitionT_(g)2 was taken as the temperature at which the transition reached halfof its height in the thermogram for the second heating procedure.

Adhesion Between the Layers

[0061] Prior to adhesive bonding, the specimen of film (300 mm long×180mm wide) of the present invention is placed on a smooth piece of card(200 mm long×180 mm wide; about 400 g/m², bleached, outer laps coated).The overlapping margins of the film are folded back onto the reverseside and secured with adhesive tape.

[0062] For adhesive bonding of the film according to the presentinvention, use is made of a standard polyester film of 12 μm thickness(e.g. Melinex 800), and a doctor device and doctor bar No. 3 fromErichsen, applying about 1.5 ml of adhesive (Novacote NC 275+ CA 12;mixing ratio: 4/1+7 parts of ethyl acetate) to the outer layer A of thefilm of the present invention. After aerating to remove the solvent, thestandard polyester film is laminated to outer layer A of the film of thepresent invention using a metal roller (width 200 mm, diameter 90 mm,weight 10 kg, to DIN EN 20 535). The lamination parameters are: Amountof adhesive: 5 +/− 1 g/m² Aeration after application of adhesive: 4 min+/− 15 s Doctor thickness (Erichsen): 3 Doctor speed level: about 133mm/s Bond curing time: 2 h at 70° C. in a circulating air drying cabinet

[0063] A 25±1 mm strip cutter is used to take specimens about 100 mm inlength. About 50 mm of composite is needed here, and 50 mm of unbondedseparate laps for securing/clamping the test specimen. The testspecimens are secured to a sheet metal support by means of double-sidedadhesive tape, by way of the entire surface of the reverse side of thefilm of the present invention (base layer B or outer layer C). The sheetwith the composite adhesively bonded thereto is clamped into the lowerclamping jaw of the tensile test machine. The clamp separation is 100mm. The unlaminated end of the standard polyester film is clamped intothe upper clamping jaw of the tensile test machine (e.g. Instron, Zwick)so that the resultant peel angle is 180°. The average peel force in N/25mm is given, rounded to one decimal place. Specimen width: 25 mmPretensioning force: 0.1N Test length: 25 mm Separation rate untilpretensioning force applied: 25 mm/min Start position: 5 mm Testdisplacement: 40 mm Sensitivity: 0.01N Separation rate: 100 mm/min

[0064] The peel force test result is equivalent to the minimum adhesionbetween the layers, since the adhesion between the adhesive and thestandard film is markedly greater. A UV lamp, for example, can be usedto demonstrate the release of the outer layer A from the base layer B ofthe film of the present invention. The UV light has a blueish appearanceif copolymer of PEN and PET is present on the adhesive and this layer isirradiated using a UV lamp.

EXAMPLES

[0065] The following examples provide further clarity of illustration ofthe invention for the skilled worker. Information on each of theproducts used (trademark and manufacturer) is given only once, and thisis then applicable to the examples which follow.

Example 1

[0066] Chips of polyethylene terephthalate and polyethylene2,6-naphthalate in a mixing ratio of 2:98 were dried at a temperature of160° C. to residual moisture below 100 ppm, and fed to the extruder forthe outer layer A.

[0067] In addition, chips of PET and PEN, in a ratio by weight of 30:70were likewise dried at 160° C. to residual moisture below 100 ppm andfed to the extruder for the intermediate layer Z.

[0068] Chips of polyethylene terephthalate were dried at a temperatureof 160° C. to residual moisture below 100 ppm and fed to the extruderfor the base layer B.

[0069] The materials were extruded at about 300° C. in each of theextruders. The melts were filtered, extruded through a coextrusion dieto give a flat film, and then placed one on top of the other as outerlayer A, intermediate layer Z, and base layer B. The composite film madefrom layers A, Z, and B was discharged over the die lip and solidifiedon a polished stainless steel chill roll. The residence time of thepolymers for the outer layer A and for the intermediate layer Z inextrusion was about 5 min. Under the conditions given, a copolymer wasproduced during the extrusion.

[0070] Coextrusion followed by stepwise longitudinal and transverseorientation was used to produce a transparent three-layer AZB film witha total thickness of 12 μm. The thickness of the outer layer A was 1.0μm and that of the intermediate layer Z was 2 μm.

[0071] Outer layer A: Outer layer A: 98% by weight of polyethylene2,6-naphthalate ( ®Polyclear P 100 prepolymer from KOSA, Offenbach) withan SV of 600, and 2% by weight of polyethylene terephthalate with SV of800. Intermediate layer Z: 70% by weight of polyethylene 2,6-naphthalate( ®Polyclear P 100 prepolymer from KOSA, Offenbach) with an SV of 600,and 30% by weight of polyethylene terephthalate with SV of 800. Baselayer B: 80% by weight of polyethylene terephthalate (4020 from KOSA,Offenbach) with SV of 800, and 20% by weight of masterbatch made from99% by weight of poly- ethylene terephthalate and 1.0% by weight ofsilica particles ( ®Sylobloc 44 H from Grace) with an average particlesize of 4.5 μm.

[0072] The individual steps of the process were: Extrusion Temperatures:Outer layer A: 300° C. Intermediate layer Z: 300° C. Base layer B: 300°C. Take-off roll 30° C. temperature Longitudinal stretching 122° C.temperature: Longitudinal stretching 4.5:1 ratio: Transverse stretching125° C. temperature: Transverse stretching 4.0:1 ratio: Settingtemperature: 230° C.

[0073] The film had the oxygen barrier required and the adhesionrequired.

Example 2

[0074] Coextrusion was used as in Example 1 to produce a four-layer AZBCfilm with a total thickness of 12 μm. The thickness of the outer layer Awas 1.5 μm, the thickness of the intermediate layer Z was 2 μm, and thethickness of the outer layer C was 1.0 μm. Outer layer A: 100% by weightof polyethylene 2,6-naphthalate (®Polyclear P 100 prepolymer from KOSA,Offenbach) with an SV of 600. Intermediate layer Z: 50% by weight ofpolyethylene 2,6-naphthalate (®Polyclear P 100 prepolymer from KOSA,Offenbach) with an SV of 600, and 50% by weight of polyethyleneterephthalate with SV of 800. Base layer B: 100% by weight ofpolyethylene terephthalate (4020 from KOSA, Offenbach) with SV of 800.Outer layer C: 80% by weight of polyethylene terephthalate with an SV of800, and 20% by weight of masterbatch made from 99.0% by weight ofpolyethylene terephthalate and 1.0% by weight of silica particles, 50%of which had an average particle size of 2.5 μm, and 50% of which had anaverage particle size of 1.0 μm.

[0075] The process conditions for all of the layers were as in Example1.

Comparative Example 1c

[0076] A film was produced by analogy with Example 8 of EP-A-0 878 297.The film had the oxygen barrier required, but the adhesion betweenlayers A and B was extremely low.

Comparative Example 2c

[0077] A film was produced by analogy with Example 1 of U.S. Pat. No.5,795,528, except that unlike in the example from the US patent therewere only 2 layers selected from PEN and PET. The film had the oxygenbarrier required, but the adhesion between layers A and B was extremelylow.

[0078] Table 3 gives the properties of the films produced in Examples 1and 2 and in the Comparative Examples 1c and 2c. TABLE 2 EthyleneEthylene Ethylene Ethylene 2,6-naphthalate terephthalate 2,6-naphthalateterephthalate units units units in units in in outer in outerintermediate intermediate layer A layer A layer Z layer Z Example (in %(in % (in % (in % No. by weight) by weight) by weight) by weight) 1 98 270 30 2 100 0 50 50 1c 100 0 0 2c 100 0 0

[0079] TABLE 3 Layer Gloss Film thicknesses Oxygen Adhesion (20°measurement Example thickness A/Z/B/C Film permeation between layersangle) No. (μm) (μm) structure (cm³/m² bar d) N/25 mm Side A Side C Haze1 12 1.0/2.0/9.0 AZB 72 1.8 200 175 1.8 2 12 1.5/2.0/7.5/1.0 AZBC 68 2.3195 180 1.9 1c 12 3.0/7.5/1.5 ABC 50 0.1 203 175 1.8 2c 12 6.0/6.0 AB 450.1 200 195 2.0

What is claimed is:
 1. A transparent, biaxially oriented polyester filmwith a base layer B, which comprises at least 80% by weight ofthermoplastic polyester, and with at least one intermediate layer Z, andwith at least one outer layer A, wherein the outer layer A is composedof a polymer, of a mixture of polymers/copolymers, or of a copolymer,which contains at least 85% by weight of ethylene 2,6-naphthalate unitsand up to 15% by weight of ethylene terephthalate units, and/or up to15% by weight of units derived from cycloaliphatic or aromatic diolsand/or dicarboxylic acids; the intermediate layer Z is composed of apolymer, of a mixture of polymers/copolymers, or of a copolymer, whichcontains at least 3% by weight of ethylene 2,6-naphthalate units, and upto 97% by weight of ethylene terephthalate units, and/or up to 97% byweight of units derived from cycloaliphatic or aromatic diols and/ordicarboxylic acids; the T_(g)2 value of the polyester film is above theT_(g)2 value of the base layer B but below the T_(g)2 value of the outerlayer A.
 2. The film as claimed in claim 1, wherein the outer layer Acontains at least 90% by weight of ethylene 2,6-naphthalate units. 3.The film as claimed in claim 1, wherein the intermediate layer Zcontains at least 5% by weight of ethylene 2,6-naphthalate units.
 4. Thefilm as claimed in claim 1, wherein the oxygen permeation of the film isbelow 85 cm³/(m²·bar·d).
 5. The film as claimed in claim 1, wherein theadhesion between the individual layers is greater than or equal to 0.5N/25 mm.
 6. The film as claimed in claim 1, wherein the outer layer Ahas a thickness above 0.3 μm.
 7. The film as claimed in claim 1, whereinthe intermediate layer Z has a thickness above 0.1 μm.
 8. The film asclaimed in claim 1, which has four layers and comprises an additionalouter layer C, arranged thereupon the base layer B, and arrangedthereupon the intermediate layer Z, and arranged thereupon the outerlayer A.
 9. The film as claimed in claim 1, wherein at least one of theouter layers has been pigmented.
 10. The film as claimed in claim 1,wherein at least one side of the film has been corona-treated.
 11. Thefilm as claimed in claim 1, wherein at least one side of the film hasbeen in-line coated.
 12. The film as claimed in claim 1, which, at leaston the outer layer A, has been metallized or ceramic-coated.
 13. Aprocess for producing the film as claimed in claim 1, encompassing thesteps producing a film from base layer B, intermediate layer Z, andouter layer(s) by coextrusion, biaxially stretching the film, andheat-setting the stretched film, which comprises carrying out thebiaxial stretching by a longitudinal stretching of the film at atemperature in the range from 80 to 130° C. and by a transversestretching in the range from 90 to 150° C. and using a longitudinalstretching ratio in the range from 2.5:1 to 6:1, and using a transversestretching ratio in the range from 3.0:1 to 5.0:1.
 14. The process asclaimed in claim 13, wherein, for heat-setting, the stretched film isheld for a period of from about 0.1 to 10 s at a temperature of from 150to 250° C.
 15. The process as claimed in claim 13, wherein cut materialarising during film production is reused as regrind in the filmproduction in amounts of up to 60% by weight, based in each case on thetotal weight of the film.